Characteristics such as oil resistance, heat resistance, cold resistance, and the like are required for the application of intake manifold gaskets. For this reason, fluororubber, NBR, silicone rubber, etc., are used as cure molding materials for such gaskets. Among these cure molding materials, fluororubber has problems in low-temperature properties and cost, silicone rubber has a problem that it is easily hydrolyzed, and NBR has a problem in that it is not suitable for sealing applications in high-temperature environments because of its extremely poor compression set characteristics.
It has been revealed that polycyclic aromatic compounds having two or more benzene rings or derivatives thereof enter into parts in which sealing materials for fuel oil are used, and adversely affect the sealing materials.
Conventionally, it was considered that polycyclic aromatic compounds adversely affected only diesel fuel automobiles using fuel having a high-boiling fraction (e.g., light oil); however, it has been recently revealed that polycyclic aromatic compounds also adversely affect automobiles using other fuel, such as gasoline fuel automobiles. A reason for this is that fuel made from crude oil that has been insufficiently refined during the production thereof may possibly be on the market and sold.
Moreover, polycyclic aromatic compounds are known to be generated during incomplete combustion of organic compounds. Various polycyclic aromatic compounds are discharged as exhaust gas. For example, exhaust gas comprising naphthalene, acenaphthene, etc., as main components is discharged from diesel fuel automobiles, and exhaust gas comprising fluoranthene, pyrene, etc., as main components is discharged from gasoline fuel automobiles (see Non-Patent Document 1). That is, there is a possibility that when automotive fuel is blown back to a part in which a sealing material is used, the sealing material is exposed to polycyclic aromatic compounds generated by combustion.
Furthermore, in order to increase the lubricity of fuel, techniques of adding polycyclic aromatic compounds, such as bicyclic aromatic compounds (e.g., naphthalene, acenaphthene, fluorene, or alkyl-substituted products thereof) and tricyclic or higher cyclic aromatic compounds (e.g., anthracene, phenanthrene, or alkyl-substituted products thereof), to fuel oil are known (see Patent Documents 1 and 2). There is a possibility that polycyclic aromatic compounds are contained as additives.
Thus, regardless of the type of fuel, it seems highly possible that sealing materials for fuel oil are exposed to polycyclic aromatic compounds.
Hydrogenated NBR, which has excellent balance of oil resistance, low-temperature properties, and cost, has been suitably used as a sealing material for general fuel oil applications. However, hydrogenated NBR is crystallized in contact with polycyclic aromatic compounds in fuel oil and thereby hardened, which causes a new problem of reducing sealing performance. In order to solve this problem of hydrogenated NBR, there is a method of reducing the acrylonitrile content of hydrogenated NBR. However, hydrogenated NBR with a low acrylonitrile content has poor oil resistance, and is thus not suitable for use in parts that are often exposed to fuel oil. In contrast, hydrogenated NBR with a high acrylonitrile content is often hardened with polycyclic aromatic compounds, and a significant reduction in sealing performance is inevitable.